Discrete Trusted Platform Module (TPM) integrated circuits (ICs) or chips such as offered by Infineon Technologies AG, Munich, Germany (World Wide Web infineon.com) may comprise hardware based security hubs that work in conjunction with software to provide a secure computing environment through cryptographic functions, key storage, digital signatures and certificates, hashes, and the like. Two major types of TPM integrated circuits currently available or being commercially developed are TPM 1.1 and TPM 1.2, which comply with different versions of the Trusted Computing Group specification, namely, version 1.1b and 1.2, respectively. Both the TPM 1.1b and TPM 1.2 integrated circuits are coupled with a standard LPC (Low Pin Count) bus.
The LPC protocol requires a transaction initiator to start a transaction with a special value on the bus called START value. The START value may provide a number of indications, including the start or stop of most types of transactions. TPM 1.1b based discrete integrated circuits use a START value of 0000. TPM 1.2 based discrete integrated circuits use an otherwise reserved START value of 0101 from standard LPC specifications. Neither a TPM 1.1b device nor a TPM 1.2 device will respond to read/write transactions unless the correct START value is driven by the LPC host controller at the start of each transaction.